Recently, disk like recording media, such as optical disks or photo magnetic disks utilizing the optical or magneto-optical signal recording/reproducing method, have been developed and presented to the market. These disk-like recording media may be classified into three types of disks according to their recording format. These are 1) a so-called read-only memory or ROM type disk, like the digital audio disk such as a compact disk or CD or video disk, in which various information signals are previously recorded by the manufacturer on the disk-like recording medium which cannot be rewritten and provided in this form to the users; 2) a so-called programmable ROM or PROM type disk, also called DRAW or write once type, in which the information signals can be written only once by the users; and 3) a so-called random access or RAM type disk, like the opto-magnetic disk, in which recorded information signals can be erased and rewritten.
These various type disks have evolved separately and at different times so that they make use of distinct formats. Because of the lack of interchangeability among these various type disks, and the resulting inconveniences to both users and manufacturers, a demand for unifying the formats has been raised by both groups. To realize a unified format, it is proposed to introduce the concept of a so-called sampling servo according to which, as with the so-called sector servo in a hard disk in the field of magnetic disks, servo signals are recorded at prescribed intervals or angular intervals on concentric or convolute tracks on the disk, these discrete servo signals being sampled and held during the disk revolution to effect continuous servo control.
This unified format includes not only a disk in which the signal recording format is limited to one of the several recording formats above, but also are in which signals are recorded in two or more recording formats on one disk. In the case of an opto-magnetic disk, for example, the servo and address signals are previously recorded by a so-called embossing, for example of mechanical pits or a pattern of projections and recesses, or bumps, and the data signals are photo-magentically recorded by the user.
It is noted that, in the reproducing the above-mentioned opto-magnetic disks, the light reflected by the recording surface is split by a beam splitter, operating on the principle of light polarization, so as to be detected by two photodetectors and the detected signals supplied from these photodetectors are summed to produce the address signals and servo signals recorded in the form of pits. The photo-magnetically recorded data signals are obtained by subtraction of the respective detected signals. The servo signals are processed by a servo signal circuit system using the aforementioned summed output signals; however, as for the address and data signals, a switching operation is required. For example switching is necessary at an input portion of the A/D converter of the reproducing signal processing system for supplying address and data signals after switching, since the signal processing system processes in a time-sharing manner.
However, when the address signals recorded in the form of pits and the photo-magnetically recorded data signals are formed consecutively in one data/address signal region 4 between servo regions 3 on one track 2, as shown in FIG. 9, it becomes necessary for the aforementioned switching to be carried out nearly instantaneously at the time of a shift from the address signal recording region to the data signal recording region during reproduction so that the switching occurs during a time interval shorter by about one digit than the interval of a window of a channel clock. In particular, when the disk is driven at a faster speed, with a higher data transfer rate, an extremely fast switching operation of the order of several nanoseconds is required, and increases the hardware load. Such a system is difficult to realize and expensive. It is noted herein that FIG. 9 shows the recording format for an arbitrary track, with the pits P as shown at line B in FIG. 9 practically formed for the recording format shown at A in FIG. 9.
It is therefore an object of the present invention to provide a disk device for at least reproducing a disk-like recording medium in which signal recording may be made in two or more physical recording formats between the servo signal regions, wherein, according to the present invention, the switching from one to the other of the physical signal recording formats at the time of reproducing the recording medium may be made with a timely margin. On the other hand, although it may be contemplated that signals be recorded in one data/address signal region 4 in accordance with one physical change, the number of bytes in one data/address signal region 4 is usually more than the number of bytes of the address data of one sector, so that, for effective utilization of the recording region, it may occasionally be desirable that signals be recorded together, with two or more format changes in one region 4. It is therefore an object of the present invention to provide a disk-like recording medium in which multiple signals may be recorded between the servo signal regions in two or more physical formats, wherein, according to the present invention, the switching from one to the other of the physical signal recording formats at the time of reproducing the recording medium may be made with a timely margin.